The phenomenon of supersensitization is well known to those skilled in the photographic art. Supersensitization is not limited to the effect of multiple sensitizing dyes themselves but also includes compounds which increase the speed of an emulsion after dye sensitization. These additives supersensitize the dye sensitizer even when the additive compound itself does not sensitize the silver halide in the spectral region in which the sensitizer is active. Upon addition of the compound, the spectral sensitivity of the dye is increased. The supersensitizer may be either increasing the absorption of light by the dye (intensifying the J-Band) or increasing the adsorption of the dye to the silver halide grain surface. These theories are described in the art (e.g., James, T. H., The Theory of the Photographic Process p. 259-261, Macmillan Publishing (N.Y. 1977), Sturge, J. M., Neblette's Handbook of Photography and Reprography, p. 92-96, Litton Education Publishing (N.Y. 1977).
Triphenylphosphine, stilbene-like moieties such as bis(triazine-2-ylamino) stilbene benzothiazole or benzoxazole type compounds, as described in U.S. Pat. No. 4,603,104 and European Patent No. 123,983 have been added to dye sensitized emulsions as speed enhancers.
Ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA) and other amine-type acetic acid compounds are not known as supersensitizers but are well known in the photographic art as chelating agents used in developer solutions during processing. This use is described in the art (e.g., U.S. Pat. No. 4,588,677).
Great Britain Patent 1,221,137 describes the use of DTPA, EDTA and other amine-type acetic acid compounds to improve emulsion sensitivity. These compounds are added during the formation of silver halide grains (precipitation) and excess compound is removed during the washing process. The patent further states that no speed enhancement is observed if the chelating agents are added after sulfur (chemical) sensitization rather than during the precipitation step.
In connection with this patent, Great Britain Patent 1,221,138 describes reducing metal spots on coated emulsion layers by the addition of these chelating agents after chemical sensitization. This invention only describes the prevention of metal spots and does not report an increase in emulsion sensitivity.
The use of mercaptotetrazoles as supersensitizers in combination with certain cyanine dyes, hydroquinones, bis(triazine-2-ylamino) stilbenes, and poly(ethylacrylate) has been described in U.S. Pat. Nos. 2,403,977; 3,266,897; 3,397,987; 3,457,078; 3,637,393 and 4,603,104.
U.K. Patent No. 691,715 discloses the improvement of light-sensitivity of colloid-silver halide emulsions by the addition of ethylenediamine tetraacetic acid (or its salts and esters) prior to the end of the emulsion digestion period for silver halide emulsions. Only small amounts are used, with a range of 0.097% to 0.91% by weight of amine to silver disclosed.
U.S. Pat. No. 3,458,316 discloses the improvement of light-sensitivity of silver halide gelatin emulsions by the addition of nitrilotriacetic acid and its water-soluble salts to emulsions prior to the precipitation of silver halide grains therein.